1. Field of the Invention
The present invention relates to a transversal filter in which the stages of an analog charge transfer device shift register, integrated on a doped semiconductor substrate, are provided with parallel inputs and evaluation or weighting networks connected to the parallel inputs, and more particularly to such a filter in which the evaluation circuits respectively exhibit an area doped opposite to the substrate, first and second input gates and a transfer gate, wherein the transfer gate is arranged directly adjacent the transfer channel of the charge transfer device shift register, the one input gate is connected to receive an input signal, the other input gate is connected to a constant direct current, the oppositely doped area is connected to a first clock pulse voltage and the transfer gate is connected to a second clock pulse voltage, and in which the output signal can be tapped at an output of the charge transfer device shift register.
2. Description of the Prior Art
A transversal filter, of the type generally mentioned above, is known from the periodical "Electronics Letters," Vol. 13, No. 5, Mar. 3, 1977, pp. 126-127. The magnitude of the individual coefficients with which the input signal is individually evaluated at each parallel input, thereby, is provided by means of the capacitance of the second input gate. Transversal filters of this species also form the subject matter of the German patent application No. P 26 43 704.7.
From the "Proceedings of the 8th Conference (1976 International) on Solid State Devices," Tokyo, 1976, published in the "Japanese Journal of Applied Physics," Vol. 16, (1977) Supplement 16-1, pp. 387-390, a transversal filter of this general type is known in which a charge shift device (CCD) shift register exhibits a plurality of transfer channels isolated from one another by means of separation diffusions, the channels being respectively individually allocated to the parallel inputs. Hereby, the transfer gate respectively provided in the evaluation circuits consist of the first shift electrode of the appertaining transfer channel. The transfer channels are brought together in a common output stage in whose area the aforementioned separation diffusions are omitted. At that point, a summing of the signal portions separately transmitted by way of the channels and arriving with varying delays takes place to form an output signal. The evaluation coefficients with which the signal supplied to the parallel inputs is applied are given by means of the gate surfaces in the individual evaluation circuits. It is, therefore, disadvantageous that the required semiconductor surface greatly increases with the plurality of the parallel inputs.
From the aforementioned article printed in the "Japanese Journal of Applied Physics," Vol. 16 (1977), Supplement 16-4 1, pp. 391-396, a further transversal filter is known which differs from the initially-mentioned species by means of a different design of the evaluation circuits and of the parallel inputs. Here, each individual evaluation circuit contains two metal-oxide-semiconductor (MOS) field effect transistors connected in series and a resistance diffusion, whereas each parallel input has a stage provided with an input diffusion, the stage being of a four-phase CCD shift register. The input diffusion is connected with a connection of the resistor and the last shift electrode is arranged adjacent to a shift electrode of a CCD shift register undertaking the summation of the signal parts. The gate of the one MOS field effect transistor of each evaluation circuit is connected with the input signal, the gate of the respectively other MOS transistor is connected with a constant current which determines the current flowing through the transistor and, therefore, the respective evaluation coefficients by means of its magnitude. Therewith, the transmission properties of the transversal filter can be electrically adjusted. However, it is disadvantageous that the current is a linear function of the constant voltage only in a small fluctuation range.
In a transversal filter of the type initially mentioned, it has already been proposed, cf. the earlier German patent application No. P 26 44 284.2, to form, for realizing an evaluation coefficient, charge amounts respectively twice in succession in the individual evaluation circuits and to input the same into the appertaining stages of the charge transfer device (CTD) shift register in order to be able to design the evaluation circuits in an economical space-saving manner.